Junk fairies are creatures that reward a good junk garden. It's considered a great honor for any Grouch to have one living in their garden. Also, a fairy is able to reward a good garden with points, and a 5 point garden is the same as a high ranking hotel.
The fairy that came to Rumpel's garden was named Schrottilde. At first Rumpel was very excited, since his junk garden could end up becoming famous; however, it turns out that a Junk Fairy is very loud (so Nils had a hard time sleeping) and very strict about junk that is too shiny to be junk, so Rumpel had to work hard to keep her happy.